In WLAN systems, the Distributed Coordination Function (DCF) is the fundamental access method for asynchronous data transfer on a best effort basis. The WLAN DCF mode is used to support contention services promoting fair access to the channel for all stations. The multiple access scheme used to achieve this is Carrier Sense Multiple Access with Carrier Avoidance (CSMA/CA). One way by which stations detect if the channel is busy is by analyzing all detected packets that are sent from other WLAN users and by detecting activity in the channel via relative signal strength from other sources. The physical carrier sensing that is performed prior to data transmission is referred to as Clear Channel Assessment (CCA).
CCA is used for transmission and reception of packets in 802.11 devices. Prior to data transmission, the device must ensure that the wireless medium is free, by using CCA. For data reception, the device only senses packets that meet the CCA criterion for a busy channel.
The 802.11 standards define different CCA modes. A commonly used CCA mode requires carrier sense and energy above the Energy Detect Threshold (EDT) before reporting that the medium is busy. More specifically, CCA reports a busy medium upon detection of a WLAN type of signal with energy above the EDT. Other CCA modes require carrier sense only, or energy above the EDT only.
A single EDT parameter is typically used to tune CCA for both transmission and reception of packets. CCA is well-tuned for transmission when:
1) The access point (AP) always senses the channel as busy when a station (STA) from its basic service set (BSS) is transmitting a packet.
2) The AP always senses the channel as busy when the STA to which it has a packet to send also senses the channel as busy due to a packet transmission from a device in a neighboring BSS. By satisfying this condition, the AP defers to external packets that would cause transmission errors.
3) The AP always senses the channel as free when the STA to which it has a packet to send senses the channel as free, even if a device from a neighboring BSS is using the channel. By satisfying this condition, unnecessary deferrals are avoided.
On the other hand, CCA is well-tuned for reception when:
1) The AP is capable of receiving packets from all STAs within the coverage area of its BSS. If the EDT parameter is set too high, the AP might not receive packets that are transmitted by a STA located at the cell edge.
2) The AP does not sense packets from devices in neighboring BSSs. If the EDT parameter is set too low, the AP might “carrier lock” onto packets that are transmitted by STAs that are located outside of its BSS or transmitted by other APs. By “locking” on external transmissions, the AP will miss any transmission from a STA in its own BSS. Such a scenario would result in a packet error, as the packet from the STA in its own BSS would collide with the external packet that the AP is receiving.
Determining the ideal EDT setting involves a trade-off between optimizing for packet transmission and optimizing for packet reception. Moreover, a dynamic method for adjusting the EDT parameter is required in order to adapt to varying network conditions (e.g., a change in the BSS size).